Compressive Testing of Metal Matrix Composites

Abstract

A newly developed direct compressive test fixture for metal matrix composites is described and evaluated in this paper. This fixture was designed to be reusable, provide accurate and reproducible data, and to eliminate premature “brooming” failures. A comparative analysis between this test method and ASTM D 3410 for compressive properties of unidirectional or cross-ply fiber-resin composites using the Illinois Institute of Technology Research Institute (IITRI) indirect compression test method was also investigated. In utilizing the direct compressive test method, the affects of specimen size and geometry were studied on a uniaxially oriented FP-alumina fiber reinforced magnesium composite. Cylindrical specimens 6.35, 9.53, and 12.7 mm in diameter, and a 9.53-mm-square specimen were tested at a constant 1/d ratio of 2.0. The tabbed specimen size and geometry that was employed in the IITRI test method was 2.54 mm thick by 6.35 mm wide by 152.4 mm long with a 12.7 mm untabbed gage length. All IITRI test specimens were tabbed by employing an improved method for bonding the aluminum tabs to the FP-alumina/magnesium surface. This tabbing procedure was developed to overcome earlier difficulties that were encountered in obtaining sufficient tabbed bond strength. All of the test specimens investigated were from a single plate of as-cast composite material that contained a fiber volume fraction of 55%. Tests were conducted with the applied load parallel to the fiber axis. Each specimen was strain gaged to provide a continuous load-strain curve to failure, to determine various materials properties, and to monitor any bending of the specimen. Data obtained included the ultimate compressive strength, the tangent modulus, the apparent proportional strain, strain to failure, and Poisson's ratio. Results from both the direct and the indirect compressive test methods were statistically compared. Finally, post test metallographic and fractographic examinations were performed to determine the principal failure modes. Final results indicate that the cylindrical specimens, regardless of their respective diameters, display the least scatter in their ultimate compressive strengths (UCS). Also, higher stress values for the cylindrical specimens was obtained; as compared to the IITRI specimens.